Failure of Ah receptor to mediate induction of cytochromes P450 in the CYP1 family in the human hepatoma line SK-Hep-1

Zearalenone and its metabolite exposure directs oestrogen metabolism towards potentially carcinogenic metabolites in human breast cancer MCF-7 cells

Zearalenone (ZEN) is produced by Fusarium species contaminating various agriculture crops. In this study, the effects of ZEN and its metabolites α-zearalenol (α-ZEL), and β-zearalenol (β-ZEL) on the formation of carcinogenic oestrogen-catechols in MCF-7 cells were investigated. To assess the effects of mycoestrogens on the activity of cytochrome P450 1A1 and CYP1B1, the rate of ethoxyresorufin O-deethylation (EROD-assay) was measured. The effects of mycoestrogens on the expression of CYP 1A1, CYP 1B1, aryl-hydrocarbon receptor (AhR), and oestrogen receptor alpha (ERα) were determined by qPCR. The catechol-O-methyltransferase (COMT) activity was measured as the ratio of the methoxy metabolites of oestradiol. Results show that mycoestrogens inhibited significantly the CYP1-dependent EROD activities. In the presence of selective inhibitors, mycoestrogens reduced CYP 1A1 and enhanced CYP 1B1 activity. Quantitative PCR analyses demonstrated the upregulation of AhR and confirmed the selective effect of mycoestrogens on CYP1 expression levels and the decline of the CYP 1A1/CYP 1B1 ratio. Mycoestrogens increased the ratio of 4-MeOE to 2-MeOE2 formation significantly (P < 0.05). Our results suggest that the tested mycoestrogens increase the production of CYP1B1-mediated oestrogen catechol metabolites, directing the biotransformation of E2 towards 4-OHE2, which has been identified earlier as a crucial factor in oestrogen-induced tumour initiation.

[Aryl-hydrocarbon receptor as a potential target for anticancer therapy]

Aryl-hydrocarbon receptor (Aryl Hydrocarbon Receptor, AHR) is a ligand-dependent transcription factor, whose functions are related to xenobiotic detoxification, response to inflammation, and maintenance of tissue homeostasis. Recent investigations suggest that AHR also plays an important role in the processes of carcinogenesis. Increased expression of AHR is observed in several types of tumors and tumor cell lines. In addition, it turned out that the composition of pharmaceutical drugs used in oncotherapy includes some ligands AHR. These facts allow us to consider an aryl-hydrocarbon receptor as a potential target for anticancer therapy, especially for the treatment of severe cancers whose treatment options are very limited or do not exist at all. In this review the examples of AHR ligands' effect on tumor cell cultures and on model mice lines with AHR-dependent response are discussed.

Induction of cytochrome P450 1A by cow milk-based formula: a comparative study between human milk and formula

During the treatment of neonatal apnea, formula-fed infants, compared to breastfed infants, show nearly three-fold increase in clearance of caffeine, a substrate of cytochrome P450 1A (CYP1A) and in part CYP3A4. However, human milk is known to contain higher concentrations of environmental pollutants than infant formula, which are potent CYP1A inducers. To gain insight into the mechanism underlying this apparent contradiction, we characterized CYP1A and CYP3A4 induction by human milk and cow milk-based infant formula. The mRNA and protein expression of CYP1A1/1A2 were significantly induced by cow milk-based formula, but not by human milk, in HepG2 cells. Luciferase reporter assay demonstrated that cow milk-based formula but not human milk activated aryl hydrocarbon receptor (AhR) significantly. The cotreatment of 3,4-dimethoxyflavone, an AhR antagonist, abolished the formula-induced CYP1A expression. In addition, AhR activation by dibenzo[a,h]anthracene, a potent AhR agonist, was significantly suppressed by infant formula and even more by human milk. In contrast, CYP3A4 mRNA expression was only mildly induced by formula and human milk. Consistently, neither formula nor human milk substantially activated pregnane X receptor (PXR). Effects of whey and soy protein-based formulas on the AhR-CYP1A and the PXR-CYP3A4 pathways were similar to those of cow milk-based formula. In conclusion, infant formula, but not human milk, enhances in vitro CYP1A expression via an AhR-mediated pathway, providing a potential mechanistic basis for the increased caffeine elimination in formula-fed infants.

Lack of CYP1A1 expression is involved in unresponsiveness of the human hepatoma cell line SK-HEP-1 to dioxin

The aryl hydrocarbon receptor (AhR) mediates a wide variety of toxic effects due to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The human hepatoma cell line SK-HEP-1 expresses AhR and ARNT. However, TCDD failed to induce CYP1A1 and XRE-dependent reporter genes in these cells. Although CYP1A1 was not induced by TCDD exposure, both CYP1B1 and AhR repressor (AhRR) were constitutively expressed. The AhR antagonist alpha-naphthoflavone altered the basal level of XRE-dependent reporter gene expression dose-dependently. As our results suggested the activation of AhR signals by putative endogenous ligands, we established SK-HEP-1-derived cell lines that stably expressed CYP1A1. The inducibility of XRE-dependent reporter genes and CYP1B1 by TCDD was restored in these cells. Our findings demonstrated the presence of endogenous ligands in SK-HEP-1 cells due to the absence of the metabolizing enzyme CYP1A1, but not CYP1B1, which allowed the constitutive expression of AhR target genes.

Identification of Metal-binding Proteins in Human Hepatoma Lines by Immobilized Metal Affinity Chromatography and Mass Spectrometry

The metalloproteome is defined as the set of proteins that have metal-binding capacity by being metalloproteins or having metal-binding sites. A different metalloproteome may exist for each metal. Mass spectrometric characterization of metalloproteomes provides valuable information relating to cellular disposition of metals physiologically and in metal-associated diseases. We examined the Cu and Zn metalloproteomes in three human hepatoma lines: Hep G2 and Mz-Hep-1, which retain many functional characteristics of normal human hepatocytes, and SK-Hep-1, which is poorly differentiated. Additionally we studied a single specimen of normal human liver and Hep G2 cells depleted in vitro of cellular copper. We used matrix-assisted laser desorption ionization and electrospray ionization quadrupole time-of-flight mass spectrometry to analyze peptide sequences of tryptic digests obtained by either in-gel digestion of metal-binding proteins or peptides on an immobilized metal affinity chromatography column loaded with either Cu or Zn. Mainly high abundance proteins were identified. Cu-binding proteins identified included enolase, albumin, transferrin, and alcohol dehydrogenase as well as certain intracellular chaperone proteins. The Cu metalloproteome was not identical to the Zn metalloproteome. Peptide binding experiments demonstrated that Cu coordination prefers the order of residues histidine > methionine > cysteine. Although the Cu metalloproteome was similar from line to line, subtle differences were apparent. Gel profiling showed more extensive variation in expression of annexin II in SK-Hep-1 and Mz-Hep-1 than in Hep G2 and normal liver tissue. Glycerylphosphorylethanolamine was identified as a post-translational modification at residue Glu-301 of elongation factor 1-alpha in Hep G2. Intracellular copper depletion was associated with loss of the glycerylphosphoryl side group. These findings suggest that post-translational modification could be affected by intracellular actions of copper. Comparison of the Cu and Zn metalloproteomes in Hep G2 with a published general proteome of Hep G2 disclosed little overlap (Seow, T. K., et al. (2001) Proteomics 1, 1249-1263). Proteins in the metalloproteomes of human hepatocytes can be identified by these methods. Variations in these metalloproteomes may have important physiological relevance.

Polycyclic aromatic hydrocarbons inhibit differentiation of human monocytes into macrophages

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (BP) are ubiquitous environmental carcinogenic contaminants exerting deleterious effects toward cells acting in the immune defense such as monocytic cells. To investigate the cellular basis involved, we have examined the consequences of PAH exposure on macrophagic differentiation of human blood monocytes. Treatment by BP markedly inhibited the formation of adherent macrophagic cells deriving from monocytes upon the action of either GM-CSF or M-CSF. Moreover, it reduced expression of macrophagic phenotypic markers such as CD71 and CD64 in GM-CSF-treated monocytic cells, without altering cell viability or inducing an apoptotic process. Exposure to BP also strongly altered functional properties characterizing macrophagic cells such as endocytosis, phagocytosis, LPS-triggered production of TNF-alpha and stimulation of allogeneic lymphocyte proliferation. Moreover, formation of adherent macrophagic cells was decreased in response to PAHs distinct from BP such as dimethylbenz(a)anthracene and 3-methylcholanthrene, which interact, like BP, with the arylhydrocarbon receptor (AhR) known to mediate many PAH effects. In contrast, benzo(e)pyrene, a PAH not activating AhR, had no effect. In addition, AhR was demonstrated to be present and functional in cultured monocytic cells, and the use of its antagonist alpha-naphtoflavone counteracted inhibitory effects of BP toward macrophagic differentiation. Overall, these data demonstrate that exposure to PAHs inhibits functional in vitro differentiation of blood monocytes into macrophages, likely through an AhR-dependent mechanism. Such an effect may contribute to the immunotoxicity of these environmental carcinogens owing to the crucial role played by macrophages in the immune defense.

Polymorphisms in the human AH receptor

The AH receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as well as induction of three cytochrome P450 enzymes and certain Phase II enzymes. In laboratory animals, genetic variations in the AHR lead to substantial differences in sensitivity to biochemical and toxic effects of TCDD and related compounds. Relatively few polymorphisms have been discovered in the human AHR gene; these occur predominantly in exon 10, a region that encodes a major portion of the transactivation domain of the receptor that is responsible for regulating expression of other genes. In human populations there is a wide range of variation in responses regulated by the AHR for example, induction of CYP1A1. Some variation in human responsiveness likely is due to genetically based variations in AHR structure. Thus far, however, only one pair of polymorphisms, those at codons 517 and 570, has been shown to have a clear cut and strong effect on the phenotype of an AHR-mediated response. The search continues for polymorphisms that alter AHR function because this receptor is a central factor in determining responses to important environmental contaminants and also plays a physiologic role in early development in mammals.